Generally, thermal recording materials comprise, on a support, a heat-sensitive recording layer containing, as main components, an electron-donating dye precursor, which is usually colorless or light-colored, and an electron-accepting compound. By application of heat to such thermal recording materials with a thermal print head, a thermal stylus, laser beam, etc., an instant reaction between the electron-donating dye precursor and the electron-accepting compound serving as a color developer occurs and thereby a recorded image is produced thereon. Such thermal recording materials are advantageous, for example, in that records can be made thereon with a relatively simple device ensuring easy maintenance and no noise generation, and therefore are widely used for measuring recorders, facsimiles, printers, computer terminals, label printers, ticket machines for passenger tickets or other tickets, and the like. Particularly in recent years, thermal recording materials are also used as receipts of gas, water, electricity and other bill payments, billing statements issued from ATMs at financial institutions, various receipts, public lotteries, thermal recording labels or tags for point of sales (POS) system, etc.
With the advance of thermal recording systems, thermal recording materials are used under severer conditions. In particular, for use under wet conditions, thermal recording materials comprising a protective layer with excellent water resistance are strongly desired.
For improvement in water resistance of thermal recording materials, as a protective layer formed on a heat-sensitive recording layer, various protective layers having different components are proposed. Among these, protective layers in which an acetoacetyl-modified polyvinyl alcohol is used have been known and are disclosed in, for example, JP-A 10-151855, JP-A 10-151856, JP-A 2004-358762, etc. As a crosslinker used in combination with the acetoacetyl-modified polyvinyl alcohol, a vinyl sulfone compound is disclosed in JP-A 2004-034436, a hydrazide compound is disclosed in JP-A 2004-249528, sebacic acid dihydrazide and dodecanedioic acid dihydrazide are disclosed in JP-A 2006-212975, an amino-containing silane coupling agent is disclosed in JP-A 2009-039874, a particular kind of aldehyde compound is disclosed in JP-A 2009-113438, and a dicarboxylic acid dihydrazide is disclosed in JP-A 2009-214422. However, such protective layers are disadvantageous in that the non-printing area of thermal recording materials may discolor under hot and humid conditions, and in that a coating liquid for forming the protective layer becomes viscous after preparation and before application, thereby hindering stable production of thermal recording materials. Thus, there is need for improvement.
Further, as thermal recording materials that excel in water resistance and discoloration resistance, thermal recording materials comprising a protective layer having crosslinks formed by a glyoxylate and a particular kind of glyoxylic acid ester derivative are disclosed in Patent Literature 1 and 2, but water resistance of these thermal recording materials is still unsatisfactory and there is need for further improvement. As a glyoxylate used as a crosslinker for acetoacetyl-modified polyvinyl alcohols, sodium glyoxylate, magnesium glyoxylate and the like are also known, but in the case of use of sodium glyoxylate, sodium ions may cause wear of thermal print heads and print failure, and in the case of use of magnesium glyoxylate, water resistance is insufficient.